The present invention relates to image forming apparatuses such as photocopiers and printers, particularly to measures to be taken against the sudden speed variation which occurs in the conveyance speed of a recording sheet or the drive speed of an image carrier resulting from the impulsive vibration produced at the time of entry of recording sheet into the roller for nipping and conveying the recording sheet, or separation of the recording sheet from the roller.
Some image forming apparatus is provided with the transfer device which transfers a toner image of an image carrier onto a recording sheet while nipping the recording sheet using a rotary member at a position opposed to the image carrier. It is known in these image forming apparatuses that the impulsive vibration produced at the time of passage (entry or separation) of the recording sheet between the image carrier and transfer device provides instantaneous speed variation to the drive speed of the image carrier, and the image in the process of being formed is adversely effected by this speed variation of the image carrier.
In this case, when thick paper is used, speed variation tends to occur due to the vibration at the time of entry or separation. This vibration may result in local but conspicuous deterioration of an image, as exemplified by transfer misalignment on the transfer section or uneven exposure on the image in the process of exposure.
The Japanese Unexamined Patent Application Publication No. 10-268595 and Japanese Unexamined Patent Application Publication No. 2007-322786 propose the measures against the instantaneous speed variation of the image carrier affected by the vibration generated at the time of recording sheet being nipped and conveyed, as described above.
The above two Japanese Unexamined Patent Application Publication documents try to adjusting the tension of the transfer belt, to give slack to the belt, to absorb vibration, and to reduce the aforementioned influence of the impulsive vibration. However, if slack is given to the belt, a slip will be produced. This is not to be preferred.
Generally, it is theoretically possible to enhance the mechanical rigidity of an apparatus to ensure that the aforementioned vibration will not occur. In actual practice, however, it is difficult to further enhance the current mechanical rigidity because of the problems with the size of the apparatus, the position and cost of a flywheel, and the overall apparatus costs.
Generally again, to avoid the aforementioned vibration, impact-absorbable flexible rollers can be used as a transfer roller and conveyance rollers of various portions to absorb impact at the time of entry or separation. In the transfer section, however, this is not preferred in the point of improving transfer efficiency and image quality.